This invention relates to voltage regulator circuits and more particularly, to a series-type voltage regulator having low power dissipation.
Operationally, a series-type voltage regulator is connected between an unregulated supply line and a load. The unregulated supply line is a variable level direct voltage source having a sufficient capability to supply the direct current (DC) demands of the load. The function of a voltage regulator is to supply the load with a constant voltage (i.e. fixed level) over a wide range of current and irrespective of voltage variations in the supply line.
To understand the operation of a series-type voltage regulator consider a regulator circuit consisting of a transistor connected in a common base configuration, with a zener diode connecting the base electrode to ground, and a resistor connected across the input collector-base junction. An unregulated supply voltage would be connected between the collector electrode and ground, and a load would be connected between emitter electrode and ground. The resistor and diode form a voltage divider circuit which maintains the base circuit at some predetermined fixed potential. Since the transistor has bias current, emitter current is supplied to the load at a voltage of approximately 0.7 volts (for silicon transistors) below the base reference voltage. The output voltage will remain relatively constant irrespective of changes in input voltage. One disadvantage of this basic circuit is that the regulator power dissipation is rather high since a relatively large bias current must flow through the resistor-diode divider network. A "worst-case design", which depends on the maximum current load and the transistor .beta. dictates a high level of bias current. And, excessive bias current results in excessive power dissipation.
An object of this invention is to provide an improved series-voltage regulator circuit having low power dissipation.